The invention relates generally to fiber optic sensors, and more particularly to a fiber optic Bragg grating sensor for detecting multiple parameters in a harsh environment.
Various fiber optic sensing devices are known and are in use for physical and chemical detection and measurement. As one example, fiber Bragg grating sensors are employed to measure parameters such as strain, vibrations, pressure, flow rate and temperature in components such as exhaust systems, nuclear reactors, combustors and compressors. Conventional fiber Bragg grating sensors employed for dynamic strain and temperature measurements include a grating structure inscribed on to a photosensitive single-mode fiber core. Typically, the fiber core is doped with Germanium oxides or co-doped with boron, fluorine, phosphors, erbium etc. Such dopants lead to formation of intra-band impurity energy levels in a silicon dioxide band gap. Furthermore, the dopants distributed in these impurity bands can be thermally excited to a conduction band and consequently induce thermal variation of the conductivity thereby causing degradation of the refractive index modulation of the fiber grating structure. Therefore, thermal stability of such sensors is limited by the operational conditions in which they may be employed.
Current fiber Bragg grating sensors are typically limited to be operated to temperatures less than about 120° C. because of temperature dependent dopant diffusion induced grating fading. Such sensors employ a doped or chemical grating that breaks down in high temperature environments. A thermal post-treatment process is typically used for stabilising the fiber grating's refractive index modulation amplitude, but such fiber Bragg grating-based sensors still have low reliability while operating above the annealing temperature.
Furthermore, as with the thermal effect, any high-energy radiations, gamma-ray and neutrons, in an environment like nuclear reactor also can prompt dopant diffusion and formation of color centers, and other defects that can lead to degradation of the fiber grating structure and the refractive index modulation.
Certain other systems employ piezoelectric-based or magnetostrictive-based sensors, such as strain gages and accelerometers, for detecting parameters such as strain and seismic signals in a harsh environment. However, such sensing devices are also limited to low-temperature environmental applications, and suffer from electromagnetic interference and radiation degradation issues.
Accordingly, there is a need for sensor that can be employed to detect a parameter, such as, temperatures or dynamic strains, in a harsh environment. Furthermore, it would be desirable to provide a sensor that can simultaneously detect multiple parameters, for example, temperature, dynamic strain, vibration, and mass flow for structural health monitoring in a harsh environment.